Probiotics in aquaculture hold promise for enhancing fish health and growth. Due to their increased specificity and affinity for their host, indigenous probiotics may offer isolated and potentially amplified benefits. This study investigated the effects of
Lactococcus lactis PH3-05, previously isolated from adults of tropical gar (
Atractosteus tropicus), on the growth, survival, digestive enzyme activity, intestinal morphology, expression of barrier and immune genes, and intestinal microbiota composition in the larvae of tropical gar. Larvae were fed with live
L. lactis PH3-05 concentrations of 10
4, 10
6, and 10
8 CFU/g for 15 days alongside a control diet without probiotics. Higher concentrations of
L. lactis PH3-05 (10
6 and 10
8 CFU/g) positively influenced larval growth, increasing hepatocyte area and enterocyte height. The 10
6 CFU/g dose significantly enhanced survival (46%) and digestive enzyme activity. Notably, the 10
8 CFU/g dose stimulated increased expression of
muc-2 and
il-10 genes, suggesting enhanced mucosal barrier function and anti-inflammatory response. Although
L. lactis PH3-05 did not significantly change the diversity, structure, or Phylum level composition of intestinal microbiota, which was constituted by Proteobacteria, Bacteroidota, Chloroflexi, and Firmicutes, an increase in
Lactobacillus abundance was observed in fish fed with 10
6 CFU/g, suggesting enhanced probiotic colonization. These results demonstrate that administering
L. lactis PH3-05 at 10
6 CFU/g promotes growth, survival, and digestive health in
A. tropicus larvae, establishing it as a promising indigenous probiotic candidate for aquaculture applications.
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